To control the output of a single induction load, such as an induction motor, an induction heating apparatus etc., there have been conventionally known electric power supply apparatuses capable of changing the frequency of an AC electric power supplied to the load (refer to, for example, Patent Document 1).
The electric power supply apparatus according to the Patent Document 1 includes a first converter which supplies a high-frequency and a second converter which supplies a medium-frequency, both converters being connected in parallel with a single induction coil. In other words, the first converter, which supplies the high-frequency, serves as a series resonance circuit, in which feedback of the medium-frequency from the second converter, which supplies medium-frequency, is reduced by a capacitor for compensating the reactive power of the induction coil. Further, a capacitor is connected in parallel with the second converter to compensate the reactive power of the induction coil, and a series circuit constituted by a reactor for restraining feedback of the high-frequency and a capacitor for additionally compensating the reactive power of the reactor is connected in series between the second converter and a common contact of the first converter and the second converter.
[Patent Document 1] Japanese Patent No. 3150968 (right column of page 2 to right column of page 3, and FIG. 3)